Researchers, including physicists from Heidelberg University, have gained new insights into dark energy and the theory of gravitation by analyzing data from the Planck satellite mission of the European Space Agency (ESA). Their results demonstrate that the standard model of cosmology remains an excellent description of the universe. Yet when the Planck data is combined with other astronomical observations, several deviations emerge. Read More →

New research offers a novel insight into the nature of dark matter and dark energy and what the future of our universe might be. Researchers in Portsmouth and Rome have found hints that dark matter, the cosmic scaffolding on which our universe is built, is being slowly erased, swallowed up by dark energy. A cosmic battle of sorts.

The new VST image of the star cluster Westerlund 1. The stars in the cluster appear red due to foreground dust blocking out their blue light. The blue stars are foreground objects and are not related to the cluster. The star W26 is in the upper left of the cluster and is surrounded by a green glow. Credit: ESO/VPHAS+ Survey/N. Wright.

An international team of astronomers has observed part of the final death throes of the largest known star in the Universe as it throws off its outer layers. The discovery, by a collaboration of scientists from the UK, Chile, Germany and the USA, is a vital step in understanding how massive stars return enriched material to the interstellar medium – the space between stars – which is necessary for forming planetary systems. The researchers publish their results in the Oxford University Press journal Monthly Notices of the Royal Astronomical Society (citation below). Read More →

The early galaxies of the universe were very different from today’s galaxies. Using new detailed studies carried out with the ESO Very Large Telescope and the Hubble Space Telescope, researchers, including members from the Niels Bohr Institute, have studied an early galaxy in unprecedented detail and determined a number of important properties such as size, mass, content of elements and have determined how quickly the galaxy forms new stars. The results are published in the scientific journal, Monthly Notices of the Royal Astronomical Society (cited below).

Quasars are among the brightest objects in the universe and can be used as lighthouses to study the universe between the quasars and Earth. Here researchers have discovered a galaxy that lies in front of a quasar and by studying the absorption lines in the light from the quasar, they have measured the elemental composition in the galaxy in great detail, despite the fact that we are looking approx. 11 billion years back in time.Graphic: Chano Birkelind, Niels Bohr Institute

“Galaxies are deeply fascinating objects. The seeds of galaxies are quantum fluctuations in the very early universe and thus, understanding of galaxies links the largest scales in the universe with the smallest. It is only within galaxies that gas can become cold and dense enough to form stars and galaxies are therefore the cradles of starsbirths”, explains Johan Fynbo, professor at the Dark Cosmology Centre at the Niels Bohr Institute at the University of Copenhagen.

Early in the universe, galaxies were formed from large clouds of gas and dark matter. Gas is the universe’s raw material for the formation of stars. Inside galaxies the gas can cool down from the many thousands of degrees it has outside galaxies. When gas is cooled it becomes very dense. Finally, the gas is so compact that it collapses into a ball of gas where the gravitational compresion heats up the matter, creating a glowing ball of gas – a star is born.

Cycle of stars

In the red-hot interior of massive stars, hydrogen and helium melt together and form the first heavier elements like carbon, nitrogen, oxygen, which go on to form magnesium, silicon and iron. When the entire core has been converted into iron, no more energy can be extracted and the star dies as a supernova explosion. Every time a massive star burns out and dies, it hence flings clouds of gas and newly formed elements out into space, where they form gas clouds that get denser and denser and eventually collapse to form new stars. The early stars contained only a thousandth of the elements found in the Sun today. In this way, each generation of stars becomes richer and richer in heavy elements.

In today’s galaxies, we have a lot of stars and less gas. In the early galaxies, there was a lot of gas and fewer stars.

“We want to understand this cosmic evolutionary history better by studying very early galaxies. We want to measure how large they are, what they weigh and how quickly stars and heavy elements are formed,” explains Johan Fynbo, who has lead the research together with Jens-Kristian Krogager, PhD student at the Dark Cosmology Centre at the Niels Bohr Institute.

Early potential for planet formation

The research team has studied a galaxy located approx. 11 billion years back in time in great detail. Behind the galaxy is a quasar, which is an active black hole that is brighter than a galaxy. Using the light from the quasar, they found the galaxy using the giant telescopes, VLT in Chile. The large amount of gas in the young galaxy simply absorbed a massive amount of the light from the quasar lying behind it. Here they could ‘see’ (i.e. via absorption) the outer parts of the galaxy. Furthermore, active star formation causes some of the gas to light up, so it could be observed directly.

Using the Hubble Space Telescope, researchers have been able to pinpoint the galaxy causing the absorption in the quasar lying behind it. In the image to the left the quasar is seen as the bright source in the center, while the absorbing galaxy, which lies in front of the quasar, is seen to the left and slightly above the quasar. In the image to the right, most of the light from the quasar is removed so the galaxy is seen more clearly. The distance between the center of the galaxy and point were the light from the quasar passes is approx. 20,000 light years, which is slightly less than the distance between the Sun and the center of the Milky Way.Credit: Hubble Telescope

With the Hubble Space Telescope they could also see the recently formed stars in the galaxy and they could calculate how many stars there were in relation to the total mass, which is comprised of both stars and gas. They could now see that the relative proportion of heavier elements is the same in the centre of the galaxy as in the outer parts and it shows that the stars that are formed earlier in the centre of the galaxy enrich the stars in the outer parts with heavier elements.

“By combining the observations from both methods – absorption and emission – we have discovered that the stars have an oxygen content equivalent to approx. 1/3 of the Sun’s oxygen content. This means that earlier generations of stars in the galaxy had already built up elements that made it possible to form planets like Earth 11 billion years ago,” conclude Johan Fynbo and Jens-Kristian Krogager.

Dark galaxies are small, gas-rich galaxies in the early Universe that are very inefficient at forming stars. They are predicted by theories of galaxy formation and are thought to be the building blocks of today’s bright, star-filled galaxies. Astronomers think that they may have fed large galaxies with much of the gas that later formed into the stars that exist today.

Because they are essentially devoid of stars, these dark galaxies don’t emit much light, making them very hard to detect. For years astronomers have been trying to develop new techniques that could confirm the existence of these galaxies. Small absorption dips in the spectra of background sources of light have hinted at their existence. However, this new study marks the first time that such objects have been seen directly.

“Our approach to the problem of detecting a dark galaxy was simply to shine a bright light on it.” explains Simon Lilly (ETH Zurich, Switzerland), co-author of the paper. “We searched for the fluorescent glow of the gas in dark galaxies when they are illuminated by the ultraviolet light from a nearby and very bright quasar. The light from the quasar makes the dark galaxies light up in a process similar to how white clothes are illuminated by ultraviolet lamps in a night club.” [1]

The team took advantage of the large collecting area and sensitivity of the Very Large Telescope (VLT), and a series of very long exposures, to detect the extremely faint fluorescent glow of the dark galaxies. They used the FORS2 instrument to map a region of the sky around the bright quasar [2] HE 0109-3518, looking for the ultraviolet light that is emitted by hydrogen gas when it is subjected to intense radiation. Because of the expansion of the Universe, this light is actually observed as a shade of violet by the time it reaches the VLT. [3]

“After several years of attempts to detect fluorescent emission from dark galaxies, our results demonstrate the potential of our method to discover and study these fascinating and previously invisible objects,” says Sebastiano Cantalupo (University of California, Santa Cruz), lead author of the study.

The team detected almost 100 gaseous objects which lie within a few million light-years of the quasar. After a careful analysis designed to exclude objects where the emission might be powered by internal star-formation in the galaxies, rather than the light from the quasar, they finally narrowed down their search to 12 objects. These are the most convincing identifications of dark galaxies in the early Universe to date.

The astronomers were also able to determine some of the properties of the dark galaxies. They estimate that the mass of the gas in them is about 1 billion times that of the Sun, typical for gas-rich, low-mass galaxies in the early Universe. They were also able to estimate that the star formation efficiency is suppressed by a factor of more than 100 relative to typical star-forming galaxies found at similar stage in cosmic history. [4]

“Our observations with the VLT have provided evidence for the existence of compact and isolated dark clouds. With this study, we’ve made a crucial step towards revealing and understanding the obscure early stages of galaxy formation and how galaxies acquired their gas”, concludes Sebastiano Cantalupo.

The MUSE integral field spectrograph, which will be commissioned on the VLT in 2013, will be an extremely powerful tool for the study of these objects.

[1] Fluorescence is the emission of light by a substance illuminated by a light source. In most cases, the emitted light has longer wavelength than the source light. For instance, fluorescent lamps transform ultraviolet radiation — invisible to us — into optical light. Fluorescence appears naturally in some compounds, such as rocks or minerals but can be also added intentionally as in detergents that contain fluorescent chemicals to make white clothes appear brighter under normal light.

[2] Quasars are very bright, distant galaxies that are believed to be powered by supermassive black holes at their centres. Their brightness makes them powerful beacons that can help to illuminate the surrounding area, probing the era when the first stars and galaxies were forming out of primordial gas.

[3] This emission from hydrogen is known as Lyman-alpha radiation, and is produced when electrons in hydrogen atoms drop from the second-lowest to the lowest energy level. It is a type of ultraviolet light. Because the Universe is expanding, the wavelength of light from objects gets stretched as it passes through space. The further light has to travel, the more its wavelength is stretched. As red is the longest wavelength visible to our eyes, this process is literally a shift in wavelength towards the red end of the spectrum — hence the name ‘redshift’. The quasar HE 0109-3518 is located at a redshift of z = 2.4, and the ultraviolet light from the dark galaxies is shifted into the visible spectrum. A narrow-band filter was specially designed to isolate the specific wavelength of light that the fluorescent emission is redshifted to. The filter was centered at around 414.5 nanometres in order to capture Lyman-alpha emission redshifted by z=2.4 (this corresponds to a shade of violet) and has a bandpass of only 4 nanometres.

[4] The star formation efficiency is the mass of newly formed stars over the mass of gas available to form stars. They found these objects would need more than 100 billion years to convert their gas into stars. This result is in accordance with recent theoretical studies that have suggested that gas-rich low-mass haloes at high redshift may have very low star formation efficiency as a consequence of lower metal content.

Analysis of data from the 10-meter South Pole Telescope is providing new support for the most widely accepted explanation of dark energy — the source of the mysterious force that is responsible for the accelerating expansion of the universe.The results also are beginning to hone in on the masses of neutrinos, the most abundant particles in the universe, which until recently were thought to be without mass.